Passive optical networks (PONs) that operate in burst mode are attractive for providing fiber-to-the-premises because, among other reasons, they enable significant equipment sharing between multiple users. Total cost of implementing a fiber-to-the-premises network, therefore, can be reduced.
A burst mode PON typically comprises several Optical Network Units (ONUs) connected to a central office through a shared optical fiber link. These ONUs communicate with the central office using a time-division-multiplexing scheme, wherein the central office allocates a transmission period to one ONU at a time to enable that ONU to transmit upstream data to the central office. In burst mode an ONU transmits its data without waiting for input from another device or waiting for an internal process to terminate before continuing the transfer of data. Since this type of transmission requires the use of transmission bandwidth for only the time while a transmitter is active, the remainder of the transmission bandwidth is available to other transmitters. As a result, a single fiber link can be shared by multiple ONUs.
As the rate at which data is transferred through the network (i.e., the transmission data rate) increases, burst mode operation becomes increasingly difficult. Specifically, as data rates exceed 10 Gbit per second, however, the number of active components in the electronic circuitry required to support such operation becomes increasingly complex, expensive, and power hungry.
One area wherein increased circuit complexity has become particularly problematic is the central office receiver that receives the upstream optical signals from the multiple ONUs. The challenges associated with receiving data at high data rates is exacerbated by the fact that the ONUs connected to the receiver are typically at different distances from the central office. Optical signals propagating through an optical fiber from the different ONUs, therefore, are subject to different losses of signal power due to the physical attributes of the fiber, different bends in the fiber, varied loss at fiber connectors within the span, and the like. In addition, the optical power launched by the transmitters of each ONU can vary significantly based on such factors as the age of the transmitter, efficiency of its optical coupling to the fiber, and differences in the electronics that drive the transmitters. As a result, the range of optical power in the optical signals received by the receiver can vary significantly. The receiver, however, must be able to generate an output signal based on all of these optical signals without incurring significant errors.